The oldest signed document in my collection...

is not by an English or British person, but by French King Francis I ("Francoys"). He was born in 1494 in Cognac and followed his cousin (Louis XII) to the throne in 1515. In 1519, he wanted to become "Emperor of the Holy Roman Empire", but lost the election against Charles, who was already King of Spain. Surrounded by Emperor Charles V´s possessions, Francis had to fight several wars during the most part of the next 20-odd years, always looking for allies, like English King Henry VIII, whom he met twice. The two were friends and enemies by turns, but most of the time they had to forget their animosities to be able to make a stand against Charles V. Francis died in March 1547, a few weeks after Henry VIII.  

This document, signed "Francoys" on 7th May 1538, in old French, ordered Guillaume Proudomme, his Finance Minister and Treasurer, to pay Martin de Trypye the sum of 5000 livres. I wasn´t able to scan the whole document, you can see the most of it here, the right quarter is missing.